This application is related to and claims priority from Japanese Patent Application No. 2001-365537, filed on Nov. 30, 2001.
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NOT APPLICABLE
The present invention relates generally to information storage systems and in particular to an information storage system having the function of mirroring, or copying, from an information storage system (hereinafter xe2x80x9ccopy source sitexe2x80x9d) comprising a plurality of information storage devices to an information storage system (hereinafter xe2x80x9ccopy destination sitexe2x80x9d) comprising one or more information storage devices.
An information storage system typically comprises a plurality of information storage devices in order to improve the response time for requests from host devices such as host computers and to expedite the storage of information. Data stored in an information storage system are processed in a way as to be copied to another information storage system for the maintenance and management thereof.
In a xe2x80x9csynchronous copyingxe2x80x9d environment, when a host device such as a server or other similar host computer issues a write request to a copy source site, the data is written to a copy destination site before the write request completes on the host device. This technique assures a high level of data currency for the host device. At any point in time, the copy source site will have the exact same data as the copy destination site.
However, because of objects such as reducing the waiting time for the host devices and information storage devices of the copy source site, a method is proposed wherein the writing of data to the copy destination site is performed without relation to the writing of new data to the copy source site. Such a method is called xe2x80x9casynchronous copying.xe2x80x9d
In the case of performing this type of asynchronous copying process, however, the copy destination information storage devices cannot determine the sequence in which to write if the sequence in which the write requests from the host devices is not known at the copy source. This is a result of the following situation. Because of dispersion or the like in the amount of data in the input instructions issued from the host devices and the capacities of the information storage devices receiving that data, the progress of processing in the information storage devices of the copy source site varies. Consequently, the sequence of write requests issued from the copy source site to the copy destination site may not be the same as the sequence of write requests issued from the host devices to the copy source site.
A function for appending the time at which the write requests were issued from the host devices to each write request (a xe2x80x9ctime stampxe2x80x9d) was developed to retain the sequence of write requests. This function is realized as an operating function of the host devices and with this time stamp, each of the information storage devices connected to the host devices acquires mutual confirmation from communication among the information storage devices regarding the time by which ends the writing received from the host devices, while also becoming able to advance the writing process.
An IBM Sysplex timer (IBM 9037) adds such a function to the mainframe that is the host device. Time stamp information for a write instruction can also be performed by the information storage device that received the instruction. It is known that the information storage device gives a time stamp using an internal clock, or the information storage device gives a time stamp using a time server, e.g., GPS (global positioning system), an internal clock of a remote operating device, or some similar device.
By applying a time stamp in this way and transferring data to be copied from the copy source site to the copy destination site, it becomes possible to recognize the order in which the data should be written to the copy destination site.
On the other hand, in order to recover from a fault in the copy source site, it is necessary to know the time up to which the writing process completed successfully. The copy source information storage devices mutually inquire how far along the processing was completed and mutually confirm the progress of the writing process. This type of mutual confirmation is called synchronization among the information storage devices.
In the field of information storage systems as discussed above, technology is also known for copying information to a system configured with separate information storage devices to protect against a fault occurring in any part of the information storage system, or a disaster or the like.
In recent years, it has been proposed to constitute storage systems of the copy destination sites also with a plurality of information storage devices to improve the response capacity for requests from a copy source information storage device and to contrive expediency in the application of stored information.
In the copy destination site, the order of writing is controlled with consciousness of the data sent from the copy source site and the order in which that writing occurred.
Further, the copy destination information storage devices also communicate with each other and carry on the writing process while making reference to the time stamp relating to the write request obtained from the copy source information storage device and synchronizing the writing process with the time to which the writing has been completed.
However, in the case where there is only one timer or the like constituting the means for acquiring the data reception order from the host device, it becomes impossible to sustain the operation to retain synchronization among the plurality of information storage devices, in the copy source site or in the copy destination site, when provisioning of the data reception time, the data reception sequence information, and so forth are suspended for some reason.
Another issue arises when the copy destination site is an information storage system comprising a plurality of information storage devices of sufficient capacity to serve as a copy source site as well. Copying among such information storage systems configured with a plurality of information storage devices is called M-to-N remote copying.
For example, another constitution considered makes possible utilization of those resources as a copy source site, and not simply as a copy destination site. This is generally called site switching, and includes making these resources available for maintenance of the copy source site, expediency in operations, or using the copy destination site as a new copy source site in the case of some fault occurring in the copy source site.
However, sufficient examination has not been made relating to M-to-N remote copying, and to this type of site switching. It is not possible to carry out site switching with the currently proposed devices connected with simple bi-directional channels.
A method and system for an information storage system comprising a plurality of storage devices receiving write requests from a plurality of host devices includes a redundant sequence generator arrangement for producing sequence information. The sequence information is associated with the write requests provide data recovery for data failures. In accordance with one aspect of the invention, a single-site configuration includes two or more sequence generators. In accordance with another aspect of the invention, a dual-site configuration includes two (or more) sequence generators distributed across the dual sites. In accordance with still another aspect of the invention, a sequence generator is provided internal to the information storage system.